Tire Size Decoder API

API ID 14561

Decode tire sizes, compare sizes with speedometer error, load/speed ratings, DOT dates.

API Documentation

Endpoints

Request
Reads the size and anything after it: service description (98H, 121/118S, 91(Y)), XL/RF, run-flat marks, M+S, load range (LR E, Load Range E, /E). Geometry is nominal and unloaded; maker-published revolutions per mile are measured under load and usually a little higher. Unreadable text is refused (400 invalid_size) rather than ignored.
Endpoint ID: 37305
GET https://zylalabs.com/api/14561/tire+size+decoder+api/37305/decode+a+tire+size
INPUT PARAMETERS

Decode a tire size — Endpoint Features

Object Description
size Required The size as printed: "225/60R16 98H", "P225/60R16 97S", "LT265/70R17 121/118S E", "33x12.50R15 108Q", "7.50R16LT", "205/65R16C 107/105T". URL-encode the slash and spaces.

Free test requests remaining: 3 of 3.


INPUT PARAMETERS

size
API EXAMPLE RESPONSE
JSON
{
    "input": "225/60R16 98H",
    "normalized": "225/60R16 98H",
    "format": "metric",
    "type": {
        "code": null,
        "name": "Passenger (Euro-metric / ISO)"
    },
    "construction": {
        "code": "R",
        "name": "Radial"
    },
    "aspectRatio": 60,
    "width": {
        "mm": 225,
        "in": 8.86
    },
    "sidewall": {
        "mm": 135,
        "in": 5.31
    },
    "rim": {
        "in": 16,
        "mm": 406.4
    },
    "diameter": {
        "mm": 676.4,
        "in": 26.63
    },
    "circumference": {
        "mm": 2125,
        "in": 83.66
    },
    "revolutions": {
        "perMile": 757,
        "perKm": 471
    },
    "service": {
        "loadIndex": {
            "index": 98,
            "kg": 750,
            "lb": 1653
        },
        "speedRating": {
            "symbol": "H",
            "kmh": 210,
            "mph": 130
        },
        "text": "98H"
    },
    "loadRange": null,
    "approximate": false,
    "notes": [],
    "warnings": [],
    "disclaimer": "Computed from the size code and public standards; nominal, unloaded dimensions. Not a safety recommendation: follow the vehicle placard (door jamb) and the tire maker's load and inflation tables."
}
Decode a tire size — CODE SNIPPETS

curl --location --request GET 'https://zylalabs.com/api/14561/tire+size+decoder+api/37305/decode+a+tire+size?size=225/60R16 98H' --header 'Authorization: Bearer YOUR_API_KEY' 


    
Request
Differences in diameter, width, sidewall and circumference (mm, inches, %), revolutions per mile and km, what the speedometer shows versus true speed for a vehicle calibrated to "from", odometer drift per 100, the 3% diameter guideline, and warnings for a lower load index or speed rating.
Endpoint ID: 37306
GET https://zylalabs.com/api/14561/tire+size+decoder+api/37306/compare+two+sizes
INPUT PARAMETERS

Compare two sizes — Endpoint Features

Object Description
from Required The original size.
to Required The new size.
speed Optional Speed shown on the speedometer (default 60 mph or 100 km/h).
unit Optional mph (default) or kmh.

Free test requests remaining: 3 of 3.


INPUT PARAMETERS

from
to
speed
unit
API EXAMPLE RESPONSE
JSON
{
    "from": "225/60R16 98H",
    "to": "235/65R16 103H",
    "diameter": {
        "from": {
            "mm": 676.4,
            "in": 26.63
        },
        "to": {
            "mm": 711.9,
            "in": 28.03
        },
        "difference": {
            "mm": 35.5,
            "in": 1.4,
            "percent": 5.25
        }
    },
    "width": {
        "from": {
            "mm": 225,
            "in": 8.86
        },
        "to": {
            "mm": 235,
            "in": 9.25
        },
        "difference": {
            "mm": 10,
            "in": 0.39,
            "percent": 4.44
        }
    },
    "sidewall": {
        "from": {
            "mm": 135,
            "in": 5.31
        },
        "to": {
            "mm": 152.8,
            "in": 6.01
        },
        "difference": {
            "mm": 17.8,
            "in": 0.7,
            "percent": 13.19
        }
    },
    "circumference": {
        "from": {
            "mm": 2125,
            "in": 83.66
        },
        "to": {
            "mm": 2236.5,
            "in": 88.05
        },
        "difference": {
            "mm": 111.5,
            "in": 4.39,
            "percent": 5.25
        }
    },
    "revolutions": {
        "from": {
            "perMile": 757,
            "perKm": 471
        },
        "to": {
            "perMile": 720,
            "perKm": 447
        },
        "difference": {
            "perMile": -37,
            "perKm": -24
        }
    },
    "speedometer": {
        "unit": "mph",
        "indicated": 60,
        "actual": 63.1,
        "errorPercent": 5.25,
        "reads": "low (you are going faster than it shows)"
    },
    "odometer": {
        "unit": "miles",
        "shown": 100,
        "actual": 105.25
    },
    "guideline": {
        "maxDiameterDifferencePercent": 3,
        "withinGuideline": false,
        "note": "Within 3% of the original overall diameter is the common rule of thumb; the vehicle maker's approved sizes come first."
    },
    "warnings": [
        "The overall diameter differs by 5.25% (more than the common 3% guideline): speedometer, ABS/stability control, gearing and clearance can be affected."
    ],
    "notes": [],
    "disclaimer": "Computed from the size code and public standards; nominal, unloaded dimensions. Not a safety recommendation: follow the vehicle placard (door jamb) and the tire maker's load and inflation tables."
}
Compare two sizes — CODE SNIPPETS

curl --location --request GET 'https://zylalabs.com/api/14561/tire+size+decoder+api/37306/compare+two+sizes?from=225/60R16 98H&to=235/65R16 103H&speed=60&unit=mph' --header 'Authorization: Bearer YOUR_API_KEY' 


    
Request
index=98 (or a dual pair 121/118 with per-axle totals) gives kg and lb per tire; minLoad=1600 (unit lb or kg) gives the lowest index that carries it; no parameters gives the whole table (0-170) and the load ranges.
Endpoint ID: 37307
GET https://zylalabs.com/api/14561/tire+size+decoder+api/37307/load+index+lookup
INPUT PARAMETERS

Load index lookup — Endpoint Features

Object Description
index Optional A load index (0-170) or a dual pair like "121/118".
minLoad Optional Load per tire to carry.
unit Optional lb (default) or kg, with minLoad.

Free test requests remaining: 3 of 3.


INPUT PARAMETERS

index
minLoad
unit
API EXAMPLE RESPONSE
JSON
{
    "index": 98,
    "kg": 750,
    "lb": 1653,
    "notes": [
        "Maximum load per tire at the maker's reference inflation pressure. A P-metric tire on a light truck, SUV or trailer is derated by 1.10 in US practice."
    ],
    "disclaimer": "Computed from the size code and public standards; nominal, unloaded dimensions. Not a safety recommendation: follow the vehicle placard (door jamb) and the tire maker's load and inflation tables."
}
Load index lookup — CODE SNIPPETS

curl --location --request GET 'https://zylalabs.com/api/14561/tire+size+decoder+api/37307/load+index+lookup?index=98' --header 'Authorization: Bearer YOUR_API_KEY' 


    
Request
symbol=H gives km/h and mph; minSpeed=125 (unit mph or kmh) gives the lowest symbol that covers it; no parameters gives the whole table, A1 to Y plus (Y) and ZR.
Endpoint ID: 37308
GET https://zylalabs.com/api/14561/tire+size+decoder+api/37308/speed+rating+lookup
INPUT PARAMETERS

Speed rating lookup — Endpoint Features

Object Description
symbol Optional A speed symbol: A1-A8, B-Y, (Y) or ZR.
minSpeed Optional Speed to cover.
unit Optional mph (default) or kmh, with minSpeed.

Free test requests remaining: 3 of 3.


INPUT PARAMETERS

symbol
minSpeed
unit
API EXAMPLE RESPONSE
JSON
{
    "symbol": "H",
    "kmh": 210,
    "mph": 130,
    "disclaimer": "Computed from the size code and public standards; nominal, unloaded dimensions. Not a safety recommendation: follow the vehicle placard (door jamb) and the tire maker's load and inflation tables."
}
Speed rating lookup — CODE SNIPPETS

curl --location --request GET 'https://zylalabs.com/api/14561/tire+size+decoder+api/37308/speed+rating+lookup?symbol=H&unit=mph' --header 'Authorization: Bearer YOUR_API_KEY' 


    
Request
The last group of the DOT number is the week and year of manufacture (4 digits since 2000; 3 before). Week 01 is the first full Sunday-Saturday week of the year (49 CFR 574.5). Returns the week's dates, the age at asOf (default today) and age advice at 5, 6 and 10 years.
Endpoint ID: 37309
GET https://zylalabs.com/api/14561/tire+size+decoder+api/37309/decode+a+dot+date+code+tire+age
INPUT PARAMETERS

Decode a DOT date code tire age — Endpoint Features

Object Description
code Required The date code ("2319") or the whole DOT number ("DOT U2LL LMLR 2319").
asOf Optional Date to measure the age at, YYYY-MM-DD (default today).

Free test requests remaining: 3 of 3.


INPUT PARAMETERS

code
asOf
API EXAMPLE RESPONSE
JSON
{
    "input": "DOT U2LL LMLR 2319",
    "dateCode": "2319",
    "format": "4 digits (2000 onward)",
    "week": 23,
    "year": 2019,
    "manufactured": {
        "weekStart": "2019-06-09",
        "weekEnd": "2019-06-15",
        "month": "2019-06"
    },
    "plantCode": "U2LL",
    "asOf": "2026-10-16",
    "age": {
        "years": 7.4,
        "months": 88
    },
    "warnings": [
        "6 years or older: many vehicle makers advise replacing tires 6 years after manufacture, and annual inspection by a professional is commonly recommended from 5 years. Check the vehicle and tire maker's guidance."
    ],
    "notes": [
        "Week 01 is the first full Sunday-Saturday week of the year (49 CFR 574.5). The date code is on one sidewall only; look on both sides."
    ],
    "disclaimer": "Computed from the size code and public standards; nominal, unloaded dimensions. Not a safety recommendation: follow the vehicle placard (door jamb) and the tire maker's load and inflation tables."
}
Decode a DOT date code tire age — CODE SNIPPETS

curl --location --request GET 'https://zylalabs.com/api/14561/tire+size+decoder+api/37309/decode+a+dot+date+code+tire+age?code=DOT U2LL LMLR 2319&asOf=2026-10-16' --header 'Authorization: Bearer YOUR_API_KEY' 


    

API Access Key & Authentication

After signing up, every developer is assigned a personal API access key, a unique combination of letters and digits provided to access to our API endpoint. To authenticate with the Tire Size Decoder API simply include your bearer token in the Authorization header.

Headers
Header Description
Authorization Required Should be Bearer access_key. See "Your API Access Key" above when you are subscribed.

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Overview

Decode tire sizes, compare sizes with speedometer error, load/speed ratings, DOT dates.

Tire Size Decoder API FAQs

Each endpoint returns structured data related to tire sizes, including specifications like width, aspect ratio, load index, speed rating, and DOT date codes. For example, the "Decode a tire size" endpoint provides detailed tire dimensions and service descriptions, while the "Compare two sizes" endpoint highlights differences in size metrics.

Key fields include "input," "normalized," "width," "diameter," "service" (load index and speed rating), and "notes." For instance, the "Decode a tire size" response includes tire dimensions in both mm and inches, along with load and speed ratings.

Parameters vary by endpoint. For "Decode a tire size," you provide the tire size string. In "Compare two sizes," you specify two tire sizes to compare. The "Load index lookup" and "Speed rating lookup" endpoints accept specific index or symbol values, while "Decode a DOT date code" requires the DOT number.

Response data is organized in a JSON format with nested objects. For example, the "Compare two sizes" endpoint includes separate sections for diameter, width, and sidewall, each detailing "from" and "to" values, along with differences in mm, inches, and percentage.

The data is computed from tire size codes and public standards, ensuring it reflects nominal, unloaded dimensions. The API emphasizes that users should follow vehicle placards and tire maker's guidelines for safety and accuracy.

Typical use cases include determining compatibility of tire sizes for vehicles, assessing speedometer accuracy based on tire changes, and evaluating tire age for safety. Users can also check load capacities and speed ratings to ensure compliance with vehicle specifications.

Users can utilize the returned data to make informed decisions about tire purchases, replacements, and maintenance. For example, comparing tire sizes helps assess potential impacts on vehicle performance, while DOT date codes inform users about tire age and safety recommendations.

If a request returns partial or empty results, users should verify the input parameters for accuracy. For instance, an invalid tire size may yield a 400 error. Users can also refer to the API documentation for guidance on valid input formats and expected outputs.

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